stellar evolution

interstellar medium (ISM)

= a star is born out of gas and dust that exists between the stars

~~ 99% of the ISM composed of interstellar gas, and of its mass about 75% from hydrogen, 25% as helium

interstellar gas consists partly of neutral atoms, molecules and charged particles (ions, electrons)

found in two terms:

cold clouds of neutral atomic / molecular hydrogen

hot ionized hydrogen near hot young gas

main sequence

name of for a continuous and distinctive band of stars that appear on a plot of stellar colour vs brightness.

the more the massive the stars, the shorter its lifespan on the main sequence

divided into upper and lower parts based on the processes that stars use to generate energy

lower mass (below 1.5 times mass of the sun)= proton-proton chain

more than 1.5 mass of the sun (upper main sequence)= nuclear fusion process can instead use atoms of carbon, nitrogen, oxygen as intermediaries in the production of helium from hydrogen atoms.

temperature gradient between the core of a star and its surface= energy steadily transported upward through the intervening layers until it is radiated away at the photosphere

convection

radiation

tends to occur in a regions with steeper temperature gradients higher opacity or both

when convection occurs in the core region it acts to stir up the helium ashes, thus maintaining the proportion of fuel needed for fusion to occur

post-main sequence

a star remains on the main sequence as long as there is hydrogen in its core that it can fuse into helium

as main sequence star ages its luminosity increases slightly, resulting in it expanding and its outer layer cooling. this explains why the main sequence is a broad band rather than a narrow line -stars move up and to the right on this band as they age

expt: our sun

move off the main sequence and up the red giant branch - fusing hydrogen into helium in hydrogran shell burning

a very short helium flash sees the start of helium core fusion and the star moves along the horizontal giant branch

eventually hydrogen core runs out and fusion stops, shutting off the outward radiation pressure

inward gravitational attraction causes the helium core to contract, converting gravitational potential energy into thermal energy

rise in temperature heats up the shell of hydrogen surrounding the core until it is hot enough to start hydrogen fusion, producing more energy than when it was a main sequence star.
this so called shell-burning causes some interesting effects.

the new increased radiation pressure actually causes the puter layer of the star to expand to maintain the pressure gradient

the expansion and cooling causes the effective temperature to drop

convection transport the energy to the outer layer of the stars from the shell burning region.
during this expansion, the star will move up and to the right on the HR diagram along the red giant branch. a G(V)- class star may end up as a high-K or low-M luminosity class 3 giant.

red giants

displays extremes of density

weakly held by gravitational force to the rest of the star and easily ejected

mass loss from a giant is typically about 10^-7 solar masses per yrs, compared with only 10^-17 solar masses per yrs currently for the sun

helium burning and the helium flash

hydrogen fusion in the shell produces more helium

this gets dumped in onto the core adding to its mass, causing it to heat up even more

when the core temperature reaches 100 mill K , the helium nuclei now have sufficient KE to overcome the strong coulombic repulsion and fuse together forming carbon-12 in a two-stage process

as three helium nuclei(ALPHA PARTICLES) are used it is called the TRIPLE ALPHA PROCESS

subsequent fusion with anther helium nucleus produces oxygen-16 nuclei. this process is the main source of the carbon and oxygen found in the universe, including that in our bodies

the horizontal branch

have a helium core-burning and hydrogen shell-burning

a solar-mass star has sufficient helium fuel for the core-burning to last for abut 100 million years

the asymptotic giant branch

energy from the helium-burning in turn heats up surrounding unused hydrogen which also starts shell burning

the giant stars expands again, possibly up to 1.5 AU equivalent to the orbit of the mars

occupying the upper-right portion of the HR diagram

exp: MIRA (O CETI)

1 solar mass AGB may have a luminosity 10 000 * that of our current sun

often more luminous in the infrared than visible wavelength

intrinsic variable stars with periods of months or a few years

nuclear reactions

proton-proton chain

CNO cycle

triple alpha